Articles and methods for use with metal surfaces



united States Patent ARTICLES AND METHODS FOR USE WITH METAL SURFACES William Y. llleakley Spokane Wasln, or to Kaiser Aluminum 0; Cllmlcnl Corporatlon, Oakland, Calif a corporation of Delaware No Drawing. A momma-11 1952 swa ms-11,093

5 Claims. (or. 21-25 The present invention relates to the prevention of water stain on the surfaces of metals susceptible thereto. More particularly, the invention relates to a fibrous material containing an effective water stain inhibitor for aluminum surfaces and to the method for producting such fibrous material. In addition, the invention is directed to the particular application of such fibrous material as a means 9f protecting stacked or nested aluminum sheet products from the development of water stain and to the method for the prevention of such stain through the medium of the inhibitor-containing fibrous material.

This application is a continuation-in-part of my copending applications S. N. 80,319 filed March 8, 1949, now abandoned, and S. N. 259,280 filed November 30, 195 i, now abandoned.

Water stain is a type of corrosion occurring on aluminum surfaces and the surfaces of other metals susceptible thereto by the action of substantially pure water under certain particular conditions. Under ordinary conditions aluminum and other metals upon which water stain occurs are unaffected by pure water. For instance, water stain does not occur on aluminum sheet roofing and siding when this material is in service and is wetted by pure water in the form of condensate or rain and where there is free access of the atmosphere to the surface. Except for a very thin oxide film formation after which the action is self-stopping, aluminum is virtually unaffected when immersed in pure water for protracted periods of time. Accordingly, water stain may be defined as that form of discoloration product which develops when moisture is present between closely contacting surfaces of metals susceptible thereto. On aluminum, this discoloration product has been identified in two forms, namely a black stain consisting of alpha monohydrate of alumina and a heavier white stain consisting of beta trihydrate of alumina. The actual mechanism of the inception and development of water stain is not completely understood, and although it is not intended to limit the present invention by any theory of action, it may be noted that it has been found that the water stain reaction appears to be accelerated in environments oratmospheres wherein there is a depletion of oxygen. On the other hand, it has also been noted that water stain resulting from moisture contacting the metal surfaces is accelerated in an oxygen enriched atmosphere or environment. Accordingly, it suflices to state that water stain occurs upon the surfaces of aluminum and other metals susceptible thereto when the surfaces are in closely contacting condition and are wetted. For example, in a stack of aluminum sheets the black and white discoloration product denominated as water stain takes the form of mirror images on the facing surfaces.

The peculiar nature of water stain and conditions under which it occurs, as above set forth, distinguish it from the usual types of corrosion common to aluminum caused by impure water or solutions containing dissolved alkalis, alkaline reacting substances, and'certain acids and acid salts and generally known as chemical corrosion. In addition, water stain is also fundamentally different from the galvanic type of corrosion on aluminum and other 2,755,167 Patented July 17, 1956 metals caused by the presence in solution of heavy metals in reducible form. As contrasted to these types of corrosion, water stain is caused solely by moisture alone when present between closely contacting surfaces of aluminum or other metals susceptible thereto. Both chemical and galvanic corrosion cause severe pitting on aluminum sheet and other materials concerned with resultant mechanical failure while water stain does not adversely affect mechanical properties.

The moisture which causes water stain inception and development may be formed by condensation of water vapor due to climatic variations in the environment, such as, sudden temperature and/or humidity changes, or by actual accidental wetting of the metal surfaces. This moisture, which may be formed between the closely contacting surfaces, or drawn between the same by capillary action gives rise to rapid formation of water stain.

As an illustration of actual conditions which are typical of those most conducive to the occurrence of water stain may be mentioned the packing of metal sheet products for shipment or storage wherein such products are in stacked or nested form and the surfaces therefore are in close contact due to the pressure exerted by the weight of the superposed articles in the stacks or nests. Any moisture or condensation between the closely contacting surfaces causes water stain to develop on these so-called inside surfaces while the exposed outer surfaces of the top and bottomsheets of the package are unaffected. The sheets upon which the water stain has developed are often rejected for use in fabrication of finished articles, or relatively expensive chemical treatment is required to remove the water stain or discoloration products prior to fabrication. In certain actual cases, water stained sheet products have been scrapped by the fabricator since the cost of the treatment to remove the water stain wasnot justified in comparison to the cost of the metal. Although, as stated above, the water stain product on the metal does not impair its mechanical properties, the adverse effect on ap pearance of the metal surface renders it unfit for many fabricated articles, particularly those which are not subjected to a metal finishing treatment. Accordingly, the occurrence of water stain in the metal industry, and in particular in the aluminum industry, has been the source of very large economic loss. It is to be noted, however, with respect to the hereinbefore mentioned stacked or nested metal sheet products that the outer surfaces of the outer sheets in 'the stacks or nests are not subject to water stain since these outer surfaces are exposed to the atmosphere and are not in closely contacting relationship with another metal sheet or interleaving material surface.

Previous attempts have been made to prevent water stain of metal surfaces, including aluminum, while shipping or storing. One of the means employed is to cover the individual surfaces of the sheet products with certain inhibited petroleum derivatives, for example, a light oil or grease, which is intended to act as a water barrier. The oil film, however, is only effective where completely continuous and permanent, a condition difficult to achieve. In addition, these coatings must be removed at the point of use, and very often specifications do not permit the use of oil or the like. Packing in containers provided with a desiccant material is unsatisfactory when applied to metal sheet products because of the high cost and cumbersome structure involved. Vapor barrier wrapping papers for the stacked or nested sheet products have been suggested, but it is extremely difficult to obtain a packaging which will not breathe, and, in any event, water vapor, which is apt to condense upon a reduction in temperature and cause the development of water stain, is present in the atmosphere inside the package. Vacuum packing is, of course, an impractical and uneconomical approach to the problem.

It has been the practice in the industry where the use of oil is not permitted by specifications to interleave the packed metal sheet products with paper. This plain or untreated interleaving paper protects the surfaces from abrasion, but offers no protection against water stain. In fact, the paper may actually promote water staining of the metal by absorption of moisture from the atmosphere and conduction of condensed water vapor to the metal surfaces through capillary action. In many instances, a severe water stain of aluminum sheets has occurred during shipment or storage when using ordinary untreated interleaving paper.

Thus, the aluminum industry has suffered serious losses because of the foregoing water stain problem and the lack of a truly effective solution which economically eliminates the occurrence of water stain.

It is therefore a primary purpose and object of the invention to provide a treated fibrous material adapted to be placed in contact with metal surfaces and which effectively acts as a water stain inhibitor for closely contacting metal surfaces susceptible thereto. Another object is to provide a process for the production of the treated water stain inhibiting fibrous material. A further object is to provide a method of preventing water stain of closely contacting metal surfaces, such as packed stacks or nested forms of metal sheet products, by means of an interleaving fibrous material containing effective amounts of a water stain inhibitor.

A specific object is to provide an interleaving paper or tissue for closely contacting aluminum or aluminum alloy sheet products, the paper containing an effective water stain inhibitor whereby the aluminum surfaces are protected against the development of water stain even though moisture is in contact with the metal surfaces. A further specific object of the invention is to provide an assembly of aluminum or aluminum alloy sheet products and treated interleaving paper and a method of packing aluminum and aluminum alloy sheet products for storage or shipment with said treated interleaving paper whereby development of water stain is prevented.

It has been discovered that the foregoing objects and other advantages may be attained by means of a fibrous interleaving material containing as the effective water stain inhibitor a soluble salt of chromic acid or a chromate.

The preparation of the water stain inhibiting fibrous material may be accomplished by any suitable means, for example, by dipping, spraying or brushing with a water solution of a salt of chromic acid or chromate, and then drying for subsequent use. As a fully acceptable alternative, soluble dichromate salts may be employed in preparing the solution for impregnating the fibrous material. However, when the impregnating solution is prepared by dissolution of solid dichromate salts, the solution is modified by addition of an appropriate amount of an alkaline reacting substance, such as a base or alkaline reacting salt, for example, an hydroxide or carbonate, as set forth in my copending application Serial No. 80,319 filed March 8, 1949.

The particular amount of alkaline substance to be added to the aqueous solution of dichromate salt is preferably that which is sufficient to produce a neutral or slightly alkaline pH, for example, 7 to 8, although any amount of such reagent which will substantially increase the initial acid pH of the dichromate solution will produce a definite increase in the water stain inhibiting action of the resulting impregnated interleaving material. Excesses of the solution-modifying base or salt which would cause residual amounts of free alkali or alkaline salt in the subsequently dried impregnated fibrous material should be avoided due to a possible detrimental effect on the interleaving material, and the possibility of an etching effect on the metal to be protected when the interleaving material and metal are assembled and exposed to the inevitable moisture which causes water stain. In general, the amount of base or alkaline salt may be readily stoichiometrically calculated in relation to the amount of dichromate employed in preparing the impregnating solution and the initial pH thereof. Thus, it is recommended the amount added be such that a solution pH of about 9 is not exceeded. Thereafter in use when the dried interleaving material becomes moistened, as for instance by condensation, the pH of the resulting solution will not exceed about 9. As stated above, the preferred pH is from about 7 to about 8.

Although, it is not intended to limit the invention to any specific mechanism or theory of action, it is believed that the effective inhibitor in the fibrous material is a chromate, or perhaps preferably, the chromate ion. Thus, when the fibrous material is impregnated with aqueous solutions of chromate salts to produce residual chromate in the dry treated paper within the broad and preferred ranges of amounts set forth below, excellent water stain inhibition are produced.

However, as indicated in my eopending application S. N. 80,319, the water stain inhibiting interleaving paper prepared by dipping the paper in simple aqueous solutions of sodium dichromate was not as effective as interleaving paper impregnated with aqueous solutions of sodium chromate. the residual amounts of both salts in the paper being comparable. Nevertheless, as taught, by my copending application S. N. 80,319. "increased protection is obtained when using a dichromate solution by the addition of a strong base, such as sodium hydroxide or other alkali hydroxide as carbonate, to the impregnating solution. In the chemistry of chromates and di chromates, it is recognized that an equilibrium of the two ions, chromate and dichromate exists in aqueous solutions, as measured by the following reactions:

Following the laws of mass action and equilibria, the actual concentrations of the two ions in any given solution, and therefore their fixed equilibrium ratio, will be dependent mainly upon the pH of the resulting solution at equilibrium, and the initial concentration of the dis solved chromate or dichromate salt (the solution temperature exerting only a minor effect). Accordingly, a chromatediaolved in distilled water produces a neutral or usually slight alkaline solution in which there is a high ratio of chromate to dichromate ions at the equilibrium pH. On the other hand, a dichromate dissolved in distilled water produces an acid solution in which there is a high ratio of dichromate to chromate ions at the equilibrium pH. Thus, in preparing chromate impregnating solutions in accordance with the invention addition of potassium chromate to distilled water of pH 5.3 resulted in equilibrium at a pH of about 7.5 by reason of the hydrolysis according to Reaction 2 above. However, potassium dichromate in equivalent amounts added to the same water produced a high ratio of dichromate to chromate ions with an equilibrium solution at pH 4.5 by reason of Reaction 1 above.

As will be seen from the foregoing, if hydroxyl ions are introduced or formed in a dichromate solution, the equilibrium represented by Reaction 2 above will be shifted to the right. As hydrogen (11+) ions are consumed, dichromate ions will continue to dissociate into chromate ions until, after sufficient alkaline reagent is added or formed, the pH of the solution has become neutral or slightly alkaline and a high ratio of chromate to dichromate iom exists at the equilibrium point of the newly established soluh'on pH.

It is believed, therefore that the dichromate solutions to which the alkaline reagent is added are, in fact, transformed into chromate solutions and that the residual salt in the dried fibrous material after treatment is a salt of chromic acid or a chromate.

Without limitation to the foregoing theory of action, the invention is intended to fully embrace the creamtion of water stain inhibiting fibrous material and the resulting article containing as efiective water stain inhibitor a soluble salt of chromic acid, or a chromate, whether prepared by impregnating the material with a simple solution of chromate or by impregnating with a dissolved solid dichromate wherein the resulting solution is modified by addition of the alkaline reacting substance. It has been determined that substantially equally effective water stain inhibition is afforded when preparing impregnating solutions by pH adjustment of dichromate solutions as by direct preparation of such solutions by dissolution of soluble chromates.

The amount of chromate or dichromate salt added in preparing the treating solution may be varied as required to obtain in the dry treated paper theefiective amounts of residual chromate content, or in some instances such as when impregnating by immersion, the time of treatment may be varied to obtain a variation within limits of residual amounts of chromate in the dry treated paper instead of varying the solution strength. It has also been found very advantageous to incorporate the chromate or chromic acid salt into the fibrous material within the desired range of residual amounts at any suitable stage in the manufacture of the fibrous material itself. It has beemfiound advantageous to use the treating solution after the fibrous web has been partially dewatered and before drying to the controlled moisture contentand calendaring. Either the so-called wet or dry saturation methods may be employed, that is, the felted web may have more or less water content when treated. Preferably, the desired residual chromate content is imparted to the paper at the stage in its manufacture wherein the tub sizing operation is usually performed on those papers which are sized. The wet treated fibrous material may be air dried or may be dried by heating by any suitable means below the charring temperature of the interleaving material.

The salts which may be utilized in preparing the water stain inhibiting interleaving fibrous material may suitably be any chromate or dichromate of a metal or cation which is water soluble, or mixtures of such salts. The degree of solubility may widely vary while obtaining excellent results. In general, however, chromates of the alkali metals, particularly sodium and potassium are preferred because of their relative economy, availability, high solubility, and freedom from the solution control aspect. Other metal and even non-metallic chromate or dichromate salts may be utilized, perhaps somewhat less economically or advantageously, for example, zinc, lithium, calcium, magnesium and ammonium chromates and/or dichromates. The use of alkaline earth metal salts generally is not contemplated due to the insolubility of the strontium and barium species and consequent difiiculty in effecting the distribution of the proper and effective residual amounts in the interleaving fibrous material. When utilizing dichromate salts, any soluble alkaline reacting substance which does not form an insoluble chromate may be employed to adjust, that is, increase the solution pH. It is preferred to use a substance having a cation common to the dichromate where hydroxides and/or carbonates are used to avoid undue complexity of the solution. However, organic bases compatible with the dichromate, i. e., not oxidizable are also contemplated.

As an illustration of the application of the water stain inhibiting fibrous sheet material of the present invention, it may be stated that in forming a pack or skid of stacked metal sheets, such as aluminum, at least one sheet of the dried treated fibrous material, for example, interleaving paper, containing the water stain inhibitor is placed between adjacent metal sheets while forming the pack, each of said sheets of fibrous material containing small but effective amounts of a water soluble salt of chromic acid whereby inception of water stain on the surfaces of the metal sheets in the stack is prevented.

It has been found that the amount of residual chromate contained in the fibrous material after treatment is important in securing the desired results, namely the prevention of inception of any water stain on the closely contacting metal surfaces interleaved by the fibrous materia As indicated in my copending application, the concentration of chromate in the treating solution in terms of percentage by weight of solution is not the best manner of expressing the required chromate content in the fibrous materials. This is due to the fact that the amount of residual chromate contained in the dry treated fibrous material may vary for the same chromate concentration in the treating solution depending upon the absorption characteristics of the paper, the particular manner in which the paper is treated by the solution, and other factors. Accordingly, it was indicated that the most appropriate expression for effecting amounts of chromate content in the fibrous material was in terms of the weight of chro mate per unit area of fibrous material. Chromate content expressed in this manner is properly definitive and significant over a certain range of weights of the fibrous material. However, the efiective range of residual chromate content per unit area of treated material varies with large variation in the weight per unit area of the fibrous material. Accordingly, it is deemed preferable to express the amounts of chromate in terms of the percentage by weight of dry treated fibrous material, which eliminates the factor of weight per unit area of paper or other fibrous material. Thus, in the specification and appended claims the amount of chromate will be indicated as percentage by weight of dry treated fibrous material, or when expressed in terms of weight of chromate per unit area of paper, such as milligrams per square inch, the weight of the paper will be indicated.

The chromate content in terms of percentage by weight of dry treated paper is easily determined from the amount of chromate per unit area of paper of a given weight as follows: Definition of ream weight of paper The weight in pounds of 500 sheets, each sheet being 24 inches by 36 inches. Therefore: One square inch of, for example, 11 pound ream weight untreated paper would weigh in milligrams Accordingly, if the residual sodium chromate content of the paper is 0.25 mg./sq. inch. then the chromate content expressed as percentage weight based on dry treated paper would be 11.56 mg./sq. in.

In contrast to the results with metal sheets uninterleaved or interleaved with untreated material, substantially any extremely low chromate content, although possibly not preventing inception of stain entirely, reduces the extent and objectionable character of the water stain product resulting from the moisture contacting the metal surfaces. In fact, in certain instances prominent local attack which occured on 24S-F aluminum alloy sheet material when interleaved with untreated paper in the presence of moisture was completely eliminated by a sodium chromate content as low as 0.01 mg.'/sq. in. of tissue for 11 pound paper. This corresponds to about 0.08% chromate based on the weight of dry treated paper. Similarly, as little as about 0.02 mg. residual chromate per square inch in 20 pound paper would be sufiicient to eliminate water stain. In general, however it is recommended that the amount of residual chromate contained in the treated fibrous material be not substantially less than about 0.75% by weight of the dry treated paper, particularly when considering the safety factor necessary for insuring pro- Hereinafter all paper weights indicated are ream weights as above-defined.

tectioa over long periods of storage. Amounts of chromate as high as about 15% or more by weight of dry treated fibrous material may be employed with completely satisfactory results in regard to elimination of water stain. However, it may be generally recommended that the residual amounts of chromate preferably should be maintained below that which may cause discoloration on the metal surfaces due to the chromate inhibitor. in general it is recommended that not substantially in excess of about 4% chromate by weight of dry treated paper be utilized in order to completely avoid any noticeable and/or objectionable discoloration caused by the chromate corrosion inhibitor itself which sometimes occur: at the higher concentrations. In addition, it serves no useful purpose and is uneconornical to employ excessively high amounts of chromate where equally good or better results are obtainable at the lower residual amounts above indicated.

Accordingly, the preferred range of amounts of residual chromate content in the fibrous material are at present considered to be not substantially less than about 0.75% and not substantially in excess of about 4% by weight of the dry treated material. Within this preferred range, it has been found that optimum results are obtained with fibrous interleaving material having a chromate content of about 2% based on the dry weight of the paper.

It has been substantiated by tests that the chromate content not substantially less than about 0.75% range gives complete water stain protection to the aluminum metal even under conditions of severe corrosion testing. In addition, utilizing not substantially more than 4% eliminates any objectionable chromate discoloration.

As the amount of corrosion inhibitor absorbed by the fibrous material from an aqueous solution thereof having a given concentration depends upon the particular type of interleaving fibrous material employed and the method of impregnation, the preferred method of stating the chromate amount is in terms of percent weight of the dry treated paper as above indicated. However, the weight of chromate may also be expressed in terms of the weight of salt per square unit of paper area. This weight will vary substantially directly as the ream weight of the paper changes, that is, the amount of chromate per unit area for a 20 pound tissue would be about twice that of a similar unit area of 10 pound tissue to afford substantially the same protection. The following table shows the amount of chromate expressed both as weight per square inch of paper area and as percentage by weight of dry treated paper.

Approximate 11 pound 1! pound Corresponding Tissue, mg. Tissue, mg. percent by wt. Na croilsq. ln. NuCrOJsq. in. of dry treated 0. 01 0. 02 0. (B 0. l5 0. 27 1.28 0. I) 0. 36 l. 71 0. 25 0. 45 2.12 0. I) 0. 55 2. 54 0. 40 0. 73 3. 86 0. 00 l. 8 7. 88 l. 8 I. 3 13. 48

It is not to be construed that the values set forth in the above chart are invariable. For instance, it has been found that the values listed in the first column for the 11 pound tissue will give excellent water stain prevention with varying paper weights ranging about pounds to about 16 pound ream weight. Accordinlly, the values of chromate concentration expressed in terms of weight per unit area of paper are definitely valid within a rather broad range of paper weights on the ream basis. This is also true for chromate values listed for the 20 pound tissue in column 2 of the chart, that is, a reasonable variation of paper weight would not cause any substantial difference 8 inthereaultsiaregardtowaterstaininhibition. Ingeneral, however, as the weight of the paper increases the chromate amount per square unit should be increased approximately proportionately.

It is to be noted that the foregoing ranges of residual chromate, broad and preferred, and the optimum residual contents are substantially the same whether the solutions employed are pH adjusted dichromate solutions, or those prepared simply by dissolving a chromate. This 00- incidence necessarily follows since the two solutions at the same neutral or alkaline pH must have the same high ratio of chromate to dichromate ions. Accordingly the residual inhibitor must be preponderantly or essentially a chromate. Even disregarding the m. anism-as presently understood, it has been determined that fibrous material containing residual inhibitor contents by impregnation with pH adjusted dichromate solutions are equally efiective in inhibiting water stain as chromate prepared solutions within the ranges above set forth, and as specifically indicated hereinbelow.

The fibrous interleaving material adapted to contact the metal surfaces may be of any suitable type, for example, paper of various grades and types, particularly kraft type of papers and tissues (a strong, generally dark brown paper produced from sulphate pulp); or it may be any other fibrous material which will protect the metal surfaces from abrasion and which has a auflicient absorp tion capacity to take up the minimum amount of chromate or dichromate solution necessary to prevent water stain. This, of course, includes woven fabric, as well as felted material. In practicing the invention, as applied to protecting packed aluminum sheet and sheet products from water stain, the types of paper normally used for interleaving, as indicated above, are preferred.

It is believed that the impregnated dried paper, containing the dry or solid chromic acid salt dispersed on the fibers, upon contacting moisture and absorbing the same, or by absorption of water vapor and subsequent condensation, or upon direct condensation of water from the atmos phere forms a protective film on the metal sheet which effectively prevents corrosion even though the moisture is in direct contact with the metal. However, it is not intended to limit the invention to the foregoing mechanism or any other theory of action, it being sutlicient to state that the interleaving material containing the chromate salt in effective amounts eliminates water stain. Results obtained with pure aluminum and various aluminum alloys were uniformly satisfactory.

The following tests and results are set forth for illustrative purposes and are not intended to constitute a limitation of the invention.

The papers used in the tests described below were prepared as follows. Strips of eleven pound kraft tissue 6" x 36" were rolled tightly around 8" test tubes and covered with other strips 6" x 18'' which served as wrappers. The wrappers were secured by means of small rubber bands at each end. The test tubes, around which the tissue strips were wrapped, were placed in tall cylinders and sodium chromate solutions of the strengths indicated in Table II below were poured over them. The period of immersion was thirty minutes, after which the test tubes were removed and allowed to dry. After immersion the outer wrappers were discarded. One inch was cut from each edge of the inner strips of tissue, leaving four inch strips from which 4" x 6" panels were cut.

The aluminum alloy panels employed in these tests were first cleaned with acetone and then wiped dry with gauze.

In the first series of tests recorded below, the samples were of 24S-F aluminum alloy which is known to be particularly susceptible to water stain. Five panels each 4" x 6" x 0.040" of this metal were interleaved with the dry 11 pound tissues prepared as above-described, and placed together to form a pack. A number of packs were prepared each interleaved with tissues containing the difsaunas Q are shown in the following table.

Table II Residual Percent Percent NasCrOi NuCrO wt. Elect an 248-! when by wt. oi solution Content, dry interleaved with Bam- Milligramsl treated ple Paper 1 sq. in. paper 0. 63 s. 88 No water stain; condition good.

0. es a 94 Do.

None None innumerable very small areas of local corrosive attack with white oxide stain roduet and occasi metal blisters.

l Observations are average performance based on four tests.

Observations are average performance based on four tests.

Examination of the above tabulated results shows that all residual sodium chromate contents used in this series of tests prevented water stain type of corrosion which occurred in the packs interleaved with untreated tissue.

In the above tests it is shown that even with 24S-F, an aluminium alloy particularly susceptible to corrosion, the chromated paper afforded ample protection against inception of water stain under conditions most conducive to its formation.

Other corrosion tests using other alloys of aluminum were also conducted. Alloys 2S-H14 and 3S-Hl4 were subjected to high humidity storage for a period of days. Five panels of sheet aluminum each 4" x 6" x .040" were placed in packs with interleaving paper between the adjacent surfaces. Packs utilizing both untreated and chromated interleaving paper (ll pound kraft tissues), as well as control packs which contained no interleaving medium, were employed in these tests. Localized pressure contact between the panels of a pack was obtained by a tie of plastic covered wire across the center of the long dimension, followed by insertion of a wooden wedge in order to simulate the pressure of shipment and storage conditions. The packs were then placed in a cabinet in which the relative humidity was about 97%. The results, after 30 days exposure, are recorded as follows:

1 Tissue used was 11 pound kraft. contained 1.62% sodium chromate by dry weight of the treated paper (0.19 mg. NaaCro /sq. in.).

Tests were performed on 24S-F and 3S-Hl4 aluminum alloys as described in connection with Table HI in which 11 pound tissues impregnated with sodium dichromate so- The chromated tissue added in amount sumcient to prdouce a solution pH of 1.5 were used as interleaving material. The residual or effective inhibitor constant in the paper and'resuits were as follows:

Table IV Residual Inhibitor, Percent b Oonditio T P D" p 7 not eat aek Panels Faint patterned stains.

flongigion good-No water stain.

Very iaint stains.

conglgion good-No water stain.

Although dichromate solutions adjusted to pH 7.5 are regarded as optimum conditions, is permissible variation of pH from not less than 5 to below about 9 is generally productive of satisfactory results (that is, the chromate content at pH not less than 5 is sufficiently increased over unmodified dichromate to improve the inhibiting eifect, while pHbelow about 9 avoids any alkaline etch of the metal). As stated above, a pH of 7 to 8 is preferred.

For comparative purposes panels interleaved with 11 pound tissue impregnated with unmodified sodium dichromate were subjected to the 30 day High humidity storage test as described with reference to Table III, above. Results were as follows:

Table V Dry aper Untreated paper Iridescent patterned stains and numerous metal blisters. irigll cent patterned stains and few tars. Iridescent tterned stains and few metal b ters. All panels gray patterned stain and few blisters Stains and numerous metal blisters.

All panels gray patterned stain with several small areas of local All ane ls grey patterned stains.

e. an en n a gray wo with s The foregoing tabulated results confirm the experimental data set forth in my copending application wherein it is shown in Table III, page 13 that simple dichromate solution impregnated interleaving paper is not completely efiective to inhibit water stain, and that (1) sodium chromate is a more effective water stain inhibtor than dichromate, and (2) increased protection is obtained when using a dichromate by addition of a base, such as sodium hydroxide or other alkali hydroxide or carbonate to the impregnating solution.

In addition to sodium hydroxide, or other alkaline reacting substances for pH adjustment of dichromate solutions are equally effective. Also other soluble chromates and dichromates are eminently satisfactory in preparing the chromate containing interleaving material. For exampklc lithium chromate impregnating solutions produced excellent water stain inhibiting results in standard 30 day High humidity storage tests as described in connection with Table III. Residual inhibitor contents of from 0.86 to 2.05% by weight of dry treated paper were employed in the tests.

Tests performed on potassium chromate impregnated 11 pound kraft tissue with residual chromate contents from .01 to 0.35 milligram per sq. in. (about 0.08 to 3% by weight of dry treated paper) were productive of water stain inhibitor results equipvalent to those obtained with atterned stains. metal blisters.)

lutions of varying strength to which sodium hydroxide was sodium chromate, using the double immersion acceleratit is important to note that the invention is not limited to the chromates and dichromates as above exemplified, but it has been determlned'tllat any soluble salt of a chromicacideflectivetoproducetheresidualchromate content in the fibrous material is suitable. Alkali metal chromates and dichromates, and the corresponding ammonium, calcium, and magnesium salts may be mentioned as eminently suitable in preparing .the fibrous interleaving material containing the desired and elective residual amounts of a soluble salt of chromic acid or chromate. Where dichromate is used to prepare the impregnating solution, the solid dichromate may be added to a solution of the pH adjusting substance until the pH is reduced to from 5 to 9 or,preferably 7 to 8. Also, two solutions may be used, for example, the fibrous material being first impregnated with the dichromate solution and then with the alkaline solution. However, direct pH adjustrnent of the previously formed dichromate solution as above set forth is preferred.

Although the invention has been described with particular reference to aluminum and aluminum alloys in the foregoing specification, it is not intended to limit the scope of the invention to this particular embodiment. The chromate containing interleaving fibrous material is eminently useful in combating the occurrence of water stain on closely contacting surfaces of all metals susceptible thereto, for example, stainless steel, chromium, nickel, etc.

Moreover it is not intended to limit the invention in its generic and operative aspects to any specific minimum residual amount of chromic acid salt, but rather to include any such amounts, however small, which are effective to substantially oflset water stain compared to that developed under conditions wherein the metal surface is not in contact with the treated fibrous material of the invention, or etfective to produce acceptable inhibitive results.

stantially less than 0.75% and not substantially in excess of about 4% by weight of dry treated fibrous material.

2.Anassemblyaccordingtoclaim l whereinthetibroussheetmaterialhpaperandthechromicacidsalt is sodium chromate.

3. A method of inhibiting water stain on aluminum sheetmaterialwhichcompriaesplacingbetweenadiacent surfacesotthemetalsheetmaterialaflbroussheetmaterial containing impregnated therein a soluble chromate in amounts not substantially less than 0.75% and not substantially in excess of 4% by weight of impregnated material.

4. Amethodaccordingtoclaimi inwhichthefibrous shtet material is paper and the chromate is an alkali metal chromate.

5. Amethodaccordingtoclaimilinwhichthefibrous sheet material is impmted with an alkali metal dichromate solution, and the pH of the 'dichromate environment then being adjusted to from about 5 to 9.

ReferneesCIsdintheflleofthispatent UNH'EDSIATES PATENTS 213,100 Eaton Mar. 11, 1879 1,628,610 Osborne May 10, 1927 2,064,325 Sutton Dec. 15, 1936 2,156,357 Simpson May 2, 1939 2,204,066 Boller June 11, 1940 FOREIGN PATENTS 6,128 Great Britain 1887 12,190 Great Britain 1888 26,533 Great Britain 1902 415,672 Great Britain Aug. 10. 1934 475,889 Great Britain Nov. 29, 1937 481,606 Great Britain Mar. 15, 1938 

3. A METHOD OF INHIBITING WATER STAIN ON ALUMINUM SHEET MATERIAL WHICH COMPRISES PLACING BETWEEN ADJACENT SURFACES OF THE METAL SHEET MATERIAL A FIBROUS SHEET MATERIAL CONTAINING IMPREGNATED THEREIN A SOLUBLE CHROMATE IN AMOUNTS NOT SUBSTANTIALLY LESS THAN 0.75% AND NOT SUBSTANTIALLY IN EXCESS OF 4% BY WEIGHT OF IMPREGNATED MATERIAL. 